Proton magnetic resonance imaging (MRI) has gained an increasing role in the diagnosis and assessment of human pathology. In patients undergoing MRI, there are numerous catheters, tubes, and other devices which are poorly seen, if they are visible at all, on the MR image. The location and course of these implanted devices is usually of great clinical importance to assure their proper function and avoid complications that malposition can cause.
Virtually all implantable catheters and similar devices are manufactured so that their locations can be determined using conventional X-ray or X-ray computed tomographic (CT) images. The techniques used to make these catheters visible on such X-ray images are not capable of rendering these catheters reliably detectable on MRI scans.
By stimulating protons in the body's molecules using the principles of nuclear magnetic resonance, images of the human body can be produced rapidly and non-invasively. Magnetic resonance images are produced from complex interactions of magnetic and radiofrequency fields without need of harmful ionizing radiation. The signal intensity or brightness of organs in a human body in a magnetic resonance image is dependent on numerous factors including intrinsic biophysical characteristics of the tissue, the radiofrequency pulse sequence or imaging technique employed to make the image, and properties of the ambient magnetic field. Tissue characteristics in the human body which impact on MRI signal intensity of a structure include proton density (number of protons per volume) and the biophysical relaxation times T1 and T2. Both normal and diseased organs have characteristic values of proton density, T1, and T2 at any given magnetic field strength. Images may be made to highlight or map tissue proton density, T1 and T2 by using different radiofrequency pulse sequences. Pulse sequences may provide different degrees of weighting of each of these characteristics for a variety of clinical purposes. Other pulse sequences have been designed to map organ motion and blood flow and are much less sensitive to differences in T1 and T2 values in the body tissues. In any given MR image there are usually a wide grey-scale range of signal intensities, from very dark to very bright. The grey-scale intensity of a given anatomic region acquired with different pulse sequences and acquisition techniques can vary substantially. For optimum conspicuity, a catheter must be manufactured so that it can be seen in dark, bright, and intermediate signal intensity anatomic regions on all types of MRI scans.
Image signal intensity is also affected by the magnetic environment. The overall strength of the imaging magnet will affect the tissue relaxation times and hence signal intensity. The strength and direction of the magnetic field gradients used in each pulse sequence to provide spatial and contrast information also significantly impact signal intensity and image appearance.
Local magnetic field non-uniformities can cause warping or even complete elimination of detectable signal. Small non-ferromagnetic metal particles can cause local image voids to occur. Ferromagnetic particles in general can cause magnetic field artifacts (MRI signal voids, with adjacent very bright signal bands, hereinafter called "imaging artifacts") which are considerably larger than the size of the particle.
Commonly used implanted medical devices are often difficult to see on MRI scans because they fail to produce sufficient contrast with respect to the surrounding body tissue or structures and/or are too small to be readily detected. Specifically this is true for foreign objects such as catheters which are introduced into the body. U.S. Pat. No. 4,572,198 so appreciated the problem and stated that if the structural portions of the catheter are simply more hydrogenous than the tissue surrounding, the catheter is detectable but a limit is placed on the available contrast. Because of the electronic noise that they introduce to the imaging apparatus, additional functional elements such as electrode wires and the like employed in U.S. Pat. No. 4,572,198 significantly degrade the magnetic resonance image often to the point of complete image obliteration. If it is usable at all, the resulting image would be clinically less diagnostic and would make accurate localization of the implanted catheter difficult if not impossible. This appears at best to be a difficult and tenuous solution to the problem. There is therefore a need for a new and improved device construction and a method so that the device inserted into the body can be more conspicuous from within different body structures yet not degrade the overall magnetic resonance image quality.
There is no intrinsic incompatibility between the techniques described to make a catheter MRI conspicuous and X-ray conspicuous. Thus, any catheter or similar device, using the technology delineated herein, may be rendered easily detected on both MRI scans and X-ray scans without degradation of either type of image.